Ducati Monster S4RS Owner's Manual page 134

Minimum tread depth
Measure the tread depth (S, fig. 59) at the point where the
tread is most worn.
It should not be less than 2 mm, and in any case not less
than the legal limit.
Important
E
Visually inspect the tyres at regular intervals for
cracks and cuts, especially on the side walls, and bulges or
large stains that indicate internal damage. Replace them if
badly damaged.
Remove any stones or other foreign bodies stuck in the
tread.
fig. 59
59